This invention relates to a station-side apparatus.
The recent prevalence of the Internet is enhancing the demand for higher-speed network communications. To meet this demand for higher-speed communications, ADSL (Asymmetric Digital Subscriber Line) or PON (Passive Optical Network) is growing. The PON includes B-PON (Broadband PON), E-PON (Ethernet PON), and G-PON (Gigabit-capable PON).
The PON is a network system which connects an accommodation station (OLT: Optical Line Terminal) installed in a vendor's station and network units (ONUs: Optical Network Units) installed in user premises; in the PON, the signal in an optical fiber connected to the OLT is split into a plurality of fibers with an optical splitter; the plurality of fibers are connected to the ONUs one by one. If a network is configured with the PON, low cost fiber installation and high-speed communications by employment of optical transmission are attained. For this reason, the PON is spreading all over the world.
Among the techniques utilizing the PON, TDM-PON (Time Division Multiplexing Passive Optical Network) is widely employed, which uses optical signals having different wavelengths in downstream transmission from the OLT to ONUs and upstream transmission from the ONUs to the OLT and further applies time-division multiplexing to the signals depending on the ONU. This TDM-PON is employed in B-PON, E-PON, G-PON, 10G-EPON, and XG-PON.
In the TDM-PON, the OLT controls the timing of sending optical signals from ONUs to prevent a conflict among the optical signals from the ONUs in upstream transmission. Specifically, the OLT sends each ONU a control frame to specify a permitted transmission period. Each ONU sends an upstream control signal and upstream data during the period specified by the received control frame.
Each ONU also sends the OLT a control frame to request a required bandwidth to the OLT, based on the data volume of a frame received from the connected terminal. In typical, the permitted transmission period for an ONU is determined through DBA (Dynamic Bandwidth Allocation) control by the OLT. The DBA control is a control method that dynamically determines the period based on the required bandwidth requested by the ONU.
Furthermore, to recover significant optical transmission loss generated in the TDM-PON, it has been generally known to employ FEC (Forward Error Correction), which is a kind of error correction coding. For example, the 10G-EPON (IEEE 802.3av standard) requires employment of Reed-Solomon (255,223) code, which is one of the FEC codes.
A system like the 10G-EPON employs the FEC regardless of transmission loss between OLT and ONUs. However, communications with short-distant ONUs generating small transmission loss do not require the FEC, although communications with long-distant ONUs generating great transmission loss do. Accordingly, in a system like the 10G-EPON, transmission efficiency will be degraded as a result of transmission of optical signals after attaching unnecessary FEC redundant codes to the signals for the short-distant ONUs.
In view of the above, a method for improving the transmission efficiency has been proposed that does not use the FEC in communications with short-distant ONUs but uses the FEC in communications with long-distant ONUs.
As to the DBA control for the TDM-PON, various control methods are known. In order to achieve fair bandwidth allocation to the ONUs, there exists a method of controlling bandwidth allocation which considers data volume information and the degree of FEC redundancy for upstream transmission data to determine upstream data transmission bandwidths to be allocated to individual subscriber's communications apparatuses (for example, refer to JP 2009-010530 A).
In the meanwhile, as a further next generation PON of the 10G-EPON or the XG-PON, there exists WDM/TDM-PON that bundles traditional TDM-PONs with a plurality of wavelengths. This WDM/TDM-PON enables still larger volume of communications by using a plurality of wavelengths.
For this WDM/TDM-PON, a technique has been proposed that dynamically changes the communication wavelength using a wavelength-tunable optical transceiver in each ONU (for example, refer to S. Kimura, “10-Gbit/s TDM-PON and over-40-Gbit/s WDM/TDM-PON systems with OPEX-effective burst-mode technologies”, OFC2009, OWH-6, March, 2009)